Although ArduinoJson is not listed in the available libraries, adding "#include <ArduinoJson.h>" seems to actually include it. The issue is that it seems to be quite outdated.
In my case, I'm trying to use it to parse JSON directly from the Serial stream, which has been possible with that library since version 5.8.0, which was released on Jan 3, 2017. Is it possible to have the library updated?
I did try including the whole thing directly in the code (which involved using the g++ preprocessor to flatten the whole library into one file), but the compiler gave a generic error message. Inspecting the network activity revealed that it was an HTTP 413, not surprising since the flattened ArduinoJson library was over 40,000 lines long. So the only way to make this work seems to be to have the library updated on the server side.